A fuel cell cogeneration system (hereinafter simply referred to as “fuel cell system”) having high electric power generation efficiency and high overall efficiency has been attracting attention as a distributed electric power generator capable of effectively utilizing energy.
The fuel cell system includes a fuel cell as a main body of an electric power generating portion. Examples of the fuel cell are a phosphoric-acid fuel cell, a molten carbonate fuel cell, an alkali aqueous solution fuel cell, a polymer electrolyte fuel cell, and a solid electrolyte fuel cell. Among these fuel cells, the phosphoric-acid fuel cell and the polymer electrolyte fuel cell (abbreviated as “PEFC”) are preferably used as the fuel cell constituting the fuel cell system since their operating temperatures during an electric power generating operation are comparatively low. Especially, in the case of the polymer electrolyte fuel cell, an electrode catalyst does not deteriorate so much, and dispersion of polymer electrolytes does not occur as compared with the phosphoric-acid fuel cell. Therefore, the polymer electrolyte fuel cell is especially preferably used in applications, such as mobile electronic devices and electric cars.
Many of the fuel cells, such as the phosphoric-acid fuel cell and the polymer electrolyte fuel cell, use hydrogen as a fuel during the electric power generating operation. However, means for supplying hydrogen necessary during the electric power generating operation is not usually disposed in the fuel cell as an infrastructure. Therefore, in order to obtain electric power by the fuel cell system including the phosphoric-acid fuel cell or the polymer electrolyte fuel cell, hydrogen as the fuel needs to be generated at an installation location of the fuel cell system. On this account, in a conventional fuel cell system, a hydrogen generator is typically disposed with the fuel cell. The hydrogen generator generates hydrogen by using, for example, steam reforming that is one of methods for generating hydrogen. In the steam reforming, a hydrocarbon raw material (material gas), such as a natural gas, a propane gas, naphtha, gasoline and kerosene and water are mixed, or an alcohol material, such as methanol and water are mixed. Then, the mixture is supplied to a reformer including a reforming catalyst. In the reformer, a steam-reforming reaction proceeds, so that the hydrogen-containing gas containing hydrogen is generated.
The hydrogen-containing gas generated in the reformer of the hydrogen generator by the steam reforming contains carbon monoxide (CO) generated as a byproduct. For example, the hydrogen-containing gas generated in the reformer of the hydrogen generator contains carbon monoxide at a concentration of about 10% to 15%. Carbon monoxide contained in the hydrogen-containing gas significantly poisons the electrode catalyst of the polymer electrolyte fuel cell. The poisoning of the electrode catalyst significantly deteriorates an electric power generating performance of the polymer electrolyte fuel cell. Therefore, in the conventional hydrogen generator, in addition to the reformer for generating the hydrogen-containing gas, a CO reducer is disposed in many cases to adequately reduce the concentration of carbon monoxide in the hydrogen-containing gas. The concentration of carbon monoxide in the hydrogen-containing gas generated in the reformer is reduced by the CO reducer up to 100 ppm or less, and preferably 10 ppm or less. The hydrogen-containing gas whose carbon monoxide is adequately removed is supplied to the fuel cell of the fuel cell system in the electric power generating operation. With this, the poisoning of the electrode catalyst is prevented in the polymer electrolyte fuel cell.
The CO reducer constituting the hydrogen generator usually includes a shift converter which causes a water gas shift reaction to proceed at a shift catalyst disposed therein to generate hydrogen and carbon dioxide from carbon monoxide and steam. The CO reducer further includes a purifier which is disposed downstream of the shift converter and has at least one of an oxidation catalyst which causes an oxidation between oxygen in the air and carbon monoxide to proceed and a methanation catalyst which causes a methanation of carbon monoxide to proceed. Using the shift converter and the purifier, the CO reducer reduces the concentration of carbon monoxide in the hydrogen-containing gas generated in the reformer up to 100 ppm or less.
The natural gas supplied to the reformer of the hydrogen generator as a raw material usually contains a slight amount of nitrogen. The concentration of nitrogen differs depending on, for example, areas where the natural gas is supplied. In a case where the natural gas containing nitrogen is supplied to the reformer of the hydrogen generator during the electric power generating operation of the fuel cell system, a chemical reaction between hydrogen generated through the steam-reforming reaction and nitrogen proceeds in the reforming catalyst included in the reformer, and as a result, ammonia may be generated. It is known that ammonia is a chemical substance which significantly deteriorates the electric power generating performance of the polymer electrolyte fuel cell. Therefore, in the case of carrying out the electric power generating operation of the fuel cell system, if the natural gas is used as the raw material and thereby ammonia is generated at a high concentration, ammonia contained in the hydrogen-containing gas generated in the hydrogen generator needs to be removed before the hydrogen-containing gas is supplied to the polymer electrolyte fuel cell.
Proposed is a fuel cell system in which an ammonia remover is disposed upstream of the polymer electrolyte fuel cell, which removes ammonia contained in the hydrogen-containing gas, and the hydrogen-containing gas whose ammonia is removed is supplied to the polymer electrolyte fuel cell (see Patent Document 1 for example).
Patent Document 1: Japanese Laid-Open Patent Application Publication 2003-31247